1. Field of the Invention
The present invention relates to a novel rare earth gallate oxide-ion conductor, having a perovskite structure. The oxide-ion conductor of the present invention exhibits very high oxide-ion conductivity or oxide-ion mixed conductivity without being substantially affected by oxygen partial pressure, and can effectively be incorporated in an electrolyte of a fuel cell, an air electrode of a fuel cell, a gas sensor such as oxygen sensor, an oxygen separating film such as electrochemical oxygen pump, a gas separator membrane, and so forth.
2. Discussion of the Background
An oxide-ion conductor owes its electrical conductivity mainly to the mobility of oxide ions (O2xe2x88x92), without substantially relying on conductivity by electrons. In general, such an oxide-ion conductor is made of a metal oxide that is doped with another metal, so as to generate O2xe2x88x92 vacancies. Attempts have been made to put such oxide-ion conductors to use in various types of materials, such as electrolytes of solid oxide (solid electrolyte) fuel cells (SOFC), gas sensors, e.g., oxygen sensors, and oxygen separator membranes of electrochemical oxygen pumps.
A typical example of such oxide-ion conductors are cubic fluorite type solid-solutions referred to as xe2x80x9cstabilized zirconiaxe2x80x9d composed of zirconium oxide (ZrO2) containing a small quantity of dissolved divalent or trivalent metal oxide, such as CaO, MgO, Y2O3, Gd2O3 or the like. The stabilized zirconia excels in heat resistance, and has conductivity that is predominantly provided by oxide ions over the entire range of oxygen partial pressure, i.e., from a pure oxygen atmosphere to a hydrogen atmosphere. Thus, the stabilized zirconia is less liable to exhibit reduction in the ion transference number (the ratio of conductivity given by the oxide ions to the overall conductivity), even under reduced oxygen partial pressure.
Zirconia oxygen sensors made of the stabilized zirconia are used for various purposes, such as control of industrial processes including steel making, air-fuel ratio control of automotive engines, and so forth. The stabilized zirconia is also used as the material of a solid oxide fuel cell (SOFC) which is being developed and which operates at temperatures around 1000xc2x0 C. It is to be noted, however, that the oxide-ion conductivity of stabilized zirconia is not so high, and tends to cause insufficiency of electrical conductivity when the temperature is lowered. For instance, the ion conductivity of a Y2O3 stabilized zirconia exhibits an ion conductivity which is as high as 10xe2x88x921 S/cm at 1000xc2x0 C. but is reduced to 10xe2x88x924 S/cm when the temperature is lowered to 500xc2x0 C. This stabilized zirconia, therefore, is usable only at high temperatures not lower than 800xc2x0 C.
Fluorite type compounds exhibits a very high oxide-ion conductivity exceeding that of stabilized zirconia. An example of such a fluorite type compound is a Bi2O3-type oxide composed of Bi2O3 containing dissolved Y2O3 in the form of a solid solution. Such a fluorite type compound, however, has a low melting point of 850xc2x0 C. or less, thus exhibiting inferior resistance to heat, although it exhibits very high levels of ion conductivity. In addition, the fluorite type compounds is not resistant to a reducing atmosphere. More specifically, when the oxygen partial pressure are lowered, n-type electron-based electrical conductivity prevails due to a change in the oxidation state of Bi3+ to Bi2+. A further reduction in the oxygen partial pressure to a level approximating a pure hydrogen atmosphere causes the compound to be reduced to the metal. The fluorite type compounds, therefore, cannot be used as a material for fuel cells.
Another kind of known fluorite type oxide-ion conductor is a ThO2 type oxide. This oxide exhibits oxide-ion conductivity much smaller than that of stabilized zirconia. In addition, electron-based electrical conduction becomes dominant so as to markedly lower the ion transference number, particularly under low oxygen partial pressures. A CeO2 type oxide, although it exhibits oxide-ion conductivity exceeding that of stabilized zirconia, permits n-type electron-based electrical conduction to prevail due to a change in the oxidation state of Ce4+ to Ce3+ when the oxygen partial pressure is reduced to 10xe2x88x9212 atm or less. Consequently, reduction of the ion transference number is also unavoidable with this type of compound.
Oxide-ion type conductors are also known that have crystalline structures other than the fluorite structure. Examples of such oxide-ion type conductors are PbWO4, LaAlO3, CaTiO3 and so forth. These conductors, however, do not have high oxide-ion conductivity and, under low oxygen partial pressure, allow semi-conduction to appear so that electron-based electrical conduction prevails, resulting in a low ion transference number.
As discussed in the foregoing, although oxide-ion conductors having higher oxide-ion conductivity than stabilized zirconia are known, such known conductors cannot suitably be used as the material of an electrolyte in solid oxide fuel cells, oxygen sensors and so forth, because of insufficiency in heat resistance and/or a large reduction in the ion transference number due to prominence of electrical conductivity provided by electrons.
Accordingly, an object of the present invention is to provide an oxide-ion conductor that has superior characteristics, such as oxide-ion conductivity greater than that of stabilized zirconia, superb heat resistance, and high oxide-ion conductivity not only at high temperatures but at low temperatures as well. Preferably, the oxidation conductor exhibits only a small reduction in the ion transference number, i.e., prominence of electrical conduction by oxide ions, over the entire range of oxygen partial pressure from that of a pure oxygen atmosphere to that of a hydrogen atmosphere, i.e., even when the oxygen partial pressure is lowered, or provide a high mixed ion conductor.
The inventors conducted an intense study to achieve the above-described object, and found that a material having high oxide-ion conductivity is obtained from rare earth gallate oxides having the perovskite structure expressed by ABO3 (wherein A is one, two or more lanthanoide-type rare earth metal(s), and B is Ga), by substituting part of the rare earth metal of the A site with an alkaline earth metal and/or substituting part of Ga atoms of the B site with a non-transition metal such as Mg, In or Al. The inventors found that a particularly high level of oxide-ion conductivity is exhibited by a compound which has the formula La0.8Sr0.2Ga0.8Mg0.2O3-w.
FIG. 1 is a graph showing the electrical conductivity of this compound in comparison with that of conventional oxide-ion conductors. From this graph, it will be seen that the compound La0.8Sr0.2Ga0.8M0.2O3-w exhibits superior electrical conductivity than those exhibited by Y2O3 stabilized zirconia and CaO stabilized zirconia, that are typical conventional zirconias. Bi2O3 type oxides exhibit electrical conductivity higher than that of the above-mentioned compound, but cannot practically be used as an oxide-ion conductor, because of the aforesaid shortcomings such as insufficiency of heat resistance and small resistance to reducing atmospheres.
The inventors have made a study to find materials which would exhibit still higher oxide-ion conductivity. As a result, the inventors have discovered that the addition of a small amount of a transition metal to the B site of the aforesaid rare earth gallate oxide provides a further improvement in the oxide-ion conductivity, thus offering satisfactorily high oxide-ion conductivity even at low temperatures.
Thus, according to the present invention, there is provided an oxide-ion conductor having the formula (1):
Ln1-xAxGa1-y-zB1yB2zO3-wxe2x80x83xe2x80x83(1)
wherein,
Ln is one, two or more elements selected from the group consisting of La, Ce, Pr, Nd and Sm;
A is one, two or more elements selected from the group consisting of Sr, Ca and Ba;
B1 is one, two or more elements selected from the group consisting of Mg, Al and In; and
B2 is one, two or more elements selected from the group consisting of Co, Fe, Ni and Cu;
and wherein the following conditions are met:
x is 0.05 to 0.3;
y is 0 to 0.29;
z is 0.01 to 0.3; and
y+z is 0.025 to 0.3; and
w corresponds to the number of oxide ion vacancies. This oxide ion conductor may also be referred to as Ln1-xAxGa1-y-zB1yB2z oxide.
In the description of the present invention, the term xe2x80x9coxide-ion conductorxe2x80x9d is used to mean electrical conductive materials that exhibit substantial oxide-ion conductivity. Thus, the term xe2x80x9coxide-ion conductorxe2x80x9d covers not only oxide-ion conductors in the narrower sense in which most of the electrical conductivity is constituted by oxide-ion conductivity, but also materials of a broader sense including materials referred to as xe2x80x9celectron-ion mixed conductorsxe2x80x9d (referred to also as xe2x80x9coxide-ion-mixed conductorsxe2x80x9d) in which both the conduction of electrons and the conduction of oxide ions constitute substantial parts of the total conduction.
In the case of the oxide-ion conductors in the narrower sense in which most of electrical conductivity is constituted by oxide-ion conductivity, the ion transference number (ratio of the electrical conductivity provided by oxide-ion conductivity to the total conductivity) is preferably 0.7 or greater, and more preferably 0.9 or greater. In case of electron-ion mixed conductor, the ion transference number preferably ranges from 0.1 to 0.7, more preferably from 0.2 to 0.6.
The present invention also provides a solid oxide fuel cell (SOFC) in which the above-mentioned oxide-ion conductor is used as the electrolyte or as the air electrode, a gas sensor using the above-mentioned oxide-ion conductor, an oxygen separator membrane of an electrochemical oxygen pump using the above-mentioned oxide-ion conductor, and a gas separator membrane made of the above-mentioned oxide-ion conductor that functions owing to a difference in gas concentration.